There is a need to display graphical information to a vehicle occupant, particularly the driver, on a large portion of a vehicle window, particularly the front windshield, to overlay the occupant's view of the outside scene. Potential uses include outlining important signs, helping the occupant to notice pedestrians, and showing road boundaries. Such applications are often referred to as augmented reality.
Potential uses for such a wide-area heads-up display for vehicle navigation were identified by Swedish researchers Martin Johansson and Marten Pettersson in an occupational study of delivery truck drivers “Eyes on the Road—Augmenting Traffic Information,” published in Proceedings of DARE 2000 on Designing Augmented Reality Environments, April 2000, pp. 147-148, (Association for Computing Machinery). Without proposing an implementation, Johansson and Pettersson found that the delivery truck driver's job would be simplified by adding visual hints to the scene viewed by the driver through the windshield.
One prior art implementation of such a wide-area heads-up display has employed a fluorescent film in or on the front windshield. The fluorescent film is excited with an ultraviolet (UV) laser beam. Both vector scanning and raster scanning methods have been demonstrated. However, this heads-up display has several significant shortcomings. The display has insufficient brightness for daytime use. There may also be concerns regarding the use of a high power UV laser in the passenger compartment of a vehicle.
Another prior art implementation of a wide-area windshield display utilizes an array of microlenses disposed inside a laminated windshield. To display an image, the windshield is illuminated by scanning with a visible laser. The driver views laser light, scattered by the microlenses. However, placing light scattering elements, such as microlenses, inside a windshield may degrade the transmittance of the windshield and cause objects viewed through the windshield to have a hazy appearance.